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FXUS62 KGSP 222343

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
743 PM EDT Mon May 22 2017

A train of Gulf low pressure systems will track through our 
area along a stalled front through Tuesday night, bringing 
thunderstorms and the potential for heavy rainfall. Another system 
will move through Wednesday, again bringing widespread heavy rain 
and below average temperatures. A drying trend will begin by the end 
of the work week and continue through early next weekend.


Weak cold front is analyzed extending roughly from the ATL metro 
area northeast into the NC Blue Ridge. The last of the strong storms 
developing across the NC Piedmont will move east of the area this 
evening even as scattered showers continue to move east across the 

More widespread convection is anticipated overnight as the first in 
a series of mid-level speed maxes and associated moisture surges is 
expected to bring another round of moderate to heavy rain with 
embedded convection. This should bring another solid inch of rain 
(with locally higher amounts) to the area by the end of Tuesday 
morning. The Flash Flood Watch looks well placed to indicate the 
threat of additional heavy rainfall. Shear profiles will be 
improving throughout this period, and with the surface boundary 
expected to establish itself somewhere near the southern border of 
our area, cannot rule out a low-end high shear/low CAPE severe 
threat southeast of I-85. However, with at least elevated convective 
elements expected, convection producing excessive rainfall rates 
will again be the larger concern. 

While guidance is not in the greatest of agreement regarding the 
details (not surprising considering the pattern), it's plausible 
that another wave of precip will waft (or begin wafting) over the 
area at some point later Tuesday. Thus, categorical pops continue 
through the afternoon. Widespread 1-2 inch amounts appear likely btw 
00Z Tue and 00Z Wed, with locally higher amounts possible likely in 
areas that see training convection. This may be enough for later 
shifts to expand the Flash Flood Watch farther north.


As of 230 PM EDT Tuesday: The short term begins 00Z Wednesday with 
the Gulf surface low exiting the area to the northeast. Pretty much 
all major models now agree on a bit of a lull in rainfall in the 
SC/NC Piedmont Tuesday night into early Wednesday morning, so pops 
overnight were trimmed down to chance in those areas. The main upper 
low/surface low will rotate through the Midwest as the main cold 
front slides through the southern Appalachians, bringing with it yet 
another round of heavy rainfall during the day on Wednesday. Many 
locations could pick up an additional inch or so of rainfall 
Wednesday, which could create some flooding issues in locations that 
present with especially wet antecedent conditions. Models continue 
to show some destabilization Wednesday afternoon, though the current 
feeling is that this is overdone, considering the widespread heavy 
rainfall moving the area during the day Wednesday. Thunder was 
included in all zones, but with relatively poor shear profiles, the 
threat of severe weather is not impressive at this time. 

By late Wednesday night, the surface front will pass through the 
forecast area, bringing with it the best chances of heavy rainfall. 
Some moisture will linger over the mountains as the upper low dives 
towards the SC/NC border and rides up the spine of the Appalachians 
as a wedge of drier air works into the SC/NC Piedmont on Thursday, 
so the best pops were maintained in the higher terrain through 
Thursday evening. Though not all that impressive, enough instability 
will linger Thursday that thunder was included in areas that feature 
pops. Convection on Thursday will be more scattered in nature than 
that associated with the better synoptic forcing earlier in the 
week. Temperatures will hover about 5-10 degrees below average.


As of 200 PM EDT Monday: The medium range picks up 00Z Friday with a 
shift towards a drier pattern. Slight chance pops linger for the NC 
mountains into early Friday morning as some moisture wraps around 
the back side of the exiting surface low, but drying during the day 
on Friday should be quick as the upper longwave trough axis/upper 
low exit off the east coast of the CONUS. The influence of surface 
high pressure and a weak upper ridge will nudge in briefly for 
another dry day on Saturday before a series of shortwaves begin to 
cross the Appalachians as upper heights fall on Sunday. The NC 
mountains will get in on precip during the day on Sunday as a 
surface low and weak cold front march eastward ahead of an 
approaching upper trough, with more widespread showers and possibly 
storms spreading through the entire forecast area Sunday evening. 
Thinking for the previous forecast was that moisture would not 
arrive in the SC/NC Piedmont zones (where better instability will be 
Sunday afternoon) until after peak diurnal heating, which would 
greatly limit any severe threat. With both global models also coming 
in a bit drier overall, I see no reason to make many adjustments to 
the current forecast.

Substantial global model differences exist from this point onwards 
in the medium range. The ECMWF brings the main surface low and cold 
front through the area Sunday night, leaving Monday mostly dry 
through the whole area. Frontal passage in the new GFS is almost a 
full 24 hours later, which presents the interesting question of 
Monday afternoon. If the GFS is correct, the front would be 
approaching the area on Monday near peak diurnal heating. Dynamics 
with the front aren't all that impressive at the moment, but decent 
instability in the SC/NC Piedmont zones coupled with the forcing 
associated with the front could present a very marginal isolated 
severe threat. Considering the greater than usual uncertainty at the 
end of the medium range and the currently dry ECMWF, forecast 
details will continue to be re-evaluated in later packages. 

Temperatures begin the medium range near average and finish about 5 
degrees above average early next week.


At KCLT and elsewhere: Convection will continue to avoid TAF sites 
this evening even as scattered showers move east across the area. 
Cannot rule out a brief restriction with the showers, but chance is 
low. Better chance for restrictions overnight as the first in a 
series of waves of low pressure developing along a stalled frontal 
boundary is expected to bring a round of widespread showers and 
embedded thunderstorms to the area after midnight. That said, timing 
any TSRA is problematic so left precip as SHRA for now. Cigs should 
also deteriorate to IFR or lower tonight around the time that the 
showers move in. Vsby may fall to IFR as well. There may be another 
lull in the precip after sunrise Tue, only to be followed by 
afternoon convection once again. Despite any lull in precip, 
restrictions will likely only slowly rise of MVFR by afternoon. 
Winds are very tricky with the waves of low pressure expected. KAVL 
should see NNW wind through the period. Elsewhere, expect winds to 
generally vary between SE and SW.

Outlook: Periods of showers and thunderstorms with associated 
restrictions will continue off and on through Thursday. Drying is 
expected to finally occur through the end of the week.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z 
KCLT       High 100%     Med   75%     High  85%     High  87%     
KGSP       High 100%     Low   46%     High  80%     Med   71%     
KAVL       High 100%     Low   58%     Med   75%     High  83%     
KHKY       High 100%     High  85%     High  80%     High  87%     
KGMU       High 100%     Med   63%     Med   75%     Med   71%     
KAND       Med   61%     Low   46%     High  85%     Med   71%     

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:


GA...Flash Flood Watch through late Wednesday night for GAZ018-026-
NC...Flash Flood Watch through late Wednesday night for NCZ064-065-
SC...Flash Flood Watch through late Wednesday night for SCZ001>014-



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